The invention generally relates to tunable oscillators and, more particularly, the invention relates to compensating for leakage in a voltage element of a tunable oscillator.
A phase locked loop (xe2x80x9cPLLxe2x80x9d) is a widely employed feedback circuit for recovering a clock signal from an input data signal. To that end, a PLL typically includes a voltage controlled oscillator (xe2x80x9cVCOxe2x80x9d) for producing an output clock signal that tracks the frequency of the input data signal. Specifically, the output dock signal generally is based upon the difference between the phase of the input data signal and, through a feedback loop, the phase of the output clock signal. The PLL thus locks onto the phase of the input data signal by tracking the output dock signal via a feedback loop. When in the steady state, the output clock signal should have a frequency that is the same as the frequency of the input data signal.
Problems can arise, however, when a PLL first receives an input data signal. In particular, prior to initial receipt of the input data signal, the VCO has no data to generate the output clock signal. Accordingly, many VCOs are configured to generate an arbitrary output clock signal for starting the clock recovery process. As known by those skilled in the art, problems often arise if the arbitrary output clock signal has a frequency that is much different from that of the input data signal. Consequently, data from the input data signal can be lost.
One way the art has responded to this problem by using voltage controlled crystal oscillators (xe2x80x9cVCXOsxe2x80x9d) instead of conventional VCOs. More particularly, VCXOs have certain material properties that enable them to generate the arbitrary output clock signal within a rated frequency range expected of the input data signal. Use of VCXOs, however, also presents a number of problems. For example, VCXOs typically are much more expensive and use more area on a circuit board than conventional VCOs. In addition, VCXOs also have a relatively limited frequency range, and often demonstrate poor jitter tolerance.
In accordance with one aspect of the invention, a tunable oscillator for producing an output signal includes an input for receiving a first voltage that is a function of the output signal, a controllable oscillator, and a voltage element that produces a second voltage and has a first leak current. The controllable oscillator produces the output signal based upon the first and second voltages, and the first leak current causes the second voltage to fluctuate. The tunable oscillator also includes a current generator operatively coupled with the input. The current generator produces a generator current that is a function of the first voltage, where the first voltage is responsive to fluctuations in the second voltage. The generator current compensates for the fluctuations in the second voltage.
In some embodiments, the generator current and a set of other currents are directed toward the voltage element. In such case, the tunable oscillator may include a node that adds the generator current and the set of other currents. The sum of all currents combined by the current combining element preferably equals, but has an opposite polarity to, the first leak current.
The first leak current may fluctuate between a zero value and a nonzero value. The tunable oscillator also may include a voltage combining element that processes the first voltage and the second voltage, and directs the processed voltages toward the controllable oscillator.
In some embodiments, the tunable oscillator also has a frequency detector for producing a comparison signal based on the comparison of the output signal and a reference signal having a reference frequency. The comparison signal is directed toward the voltage element. The output signal has an output frequency, and the comparison signal has a first maximum amplitude when the difference between the output frequency and the reference frequency exceeds a preset amount. The comparison signal has a second maximum amplitude when the difference between the output frequency and the reference frequency does not exceed the preset amount. The first maximum amplitude is greater than the second maximum amplitude.
Another aspect of the invention includes a system for compensating for potential fluctuations in a voltage element of a tunable oscillator. The voltage element produces a first tuning voltage that fluctuates in response to the fluctuations, while the tunable oscillator produces an output signal with an output characteristic that is a function of the first tuning voltage. The system includes a current generator that produces a generator current based upon a second tuning voltage, and a current delivery node that directs the generator current toward the voltage element. The second tuning voltage is a function of the output characteristic, while the first tuning voltage is a function of the generator current to dynamically offset the fluctuations.
In some embodiments, a set of other currents are directed toward the voltage element. The current delivery node preferably includes a current combining element that combines the generator current with the set of other currents. In other embodiments, the output characteristic may be at least one of an output frequency and an output phase.
In some embodiments, the voltage element includes a capacitor that stores a voltage, and the compensation current controls the voltage of the capacitor in response to the fluctuations. The fluctuations may be caused by a leakage current of the voltage element, and the current generator may include a transconductance amplifier.